Nowadays, water flow sensors provided on tankless gas water heater for detecting water flow include mainly ones of axial-flow type and ones of lateral-flow type. Both of them are configured to detect water flow by a Hall element in accordance with pulse signal generated due to polarity change of a magnet, while a position of the magnet is variable with rotation of a blade wheel impelled by water.
The axial-flow type water flow sensor is provided at a water inlet with a cyclone formed by a plurality of turbine-like fixed blades. One-way flow of water through the cyclone is converted into a rotational flow, which brings magnetic blades of a water wheel into rotation, while a Hall element is configured to detect the water flow in accordance with pulse signal generated due to polarity change of the magnetic blades. This type of water flow sensor has advantages including: there is a good linear relation between the water flow and a rotational speed of the magnetic blades, while the rotational speed is stable; the blade wheel is under uniform stress in radial direction, and is merely under impelling stress from waterflow in axial direction, thus there may be a small radial wear between a shaft of the blade wheel and a bearing after long-term use, while an axial wear may not cause adverse effects on an operation thereof since there is enough wear allowance remained on an upper bearing of the rotational shaft of the blade wheel, which guarantees a long lifespan and a stable operation. However, disadvantages of this type of water flow sensor includes: irony impurities in tap water may be adhered to the magnetic water wheel; even if providing a high-strength magnet at the water inlet to make an attempt to remove ferromagnetic impurities from the water, a lot of irony impurities may still be adhered to the magnetic water wheel after long term operation, so that the water wheel may be jammed and unable to run smoothly. Meanwhile, a thickness of the blade of the magnetic water wheel is required to be more than 2 mm, thus there may be a relative large impact force in axial direction applied on the water wheel by water during operation.
The lateral-flow type water flow sensor is provided with a passage for water inflow and outflow at one side of blades of a water wheel, and the water wheel is rotated by impact from the waterflow. A magnet located at another side of a shaft of the water wheel and separated from the water wheel is brought into rotation by the water wheel, while a Hall element may detect a water flow according to polarity change of the magnet which is in synchronous rotation with the water wheel. Its advantage lies in that the magnet can be separated from the water wheel and the water passage. However, a linear relation between a rotation speed of the water wheel and a water flow is not good enough, so that detection accuracy may be affected. Moreover, since the blades of the water wheel are impelled by the waterflow from a single side, wear may be apt to occur between the shaft of the water wheel and a bearing which largely suffer from force in radial direction of the water wheel, and thus the lifespan may be shortened.
For those two types of water flow sensors mentioned above, the water wheel thereof may have its mass increased due to presence of the magnet, and the inertia may be increased, thus a response speed to a transient variation of the water flow may be decreased.